This invention relates to a rolling bearing apparatus for a wheel used mainly in an automobile.
There is known a conventional wheel rolling bearing apparatus, for supporting a driving wheel, in which a drive shaft connected to an engine is coupled to a central hole in an inner shaft for the mounting a wheel thereon so that the drive shaft and the inner shaft can rotate in unison. Such a wheel bearing apparatus is so constructed that an inner ring of a rolling bearing is mounted onto an outer periphery of the inner shaft from the inner side (that is, that side facing away from the wheel-mounting side) as described in Japanese Utility Model Registration No. 2,532,672. The drive shaft includes a larger-diameter portion formed at its inner side portion, and a smaller-diameter portion formed at its outer side portion (that is, at the wheel mounting-side), and this smaller-diameter portion is fitted in the center hole of the inner shaft. A spline formed on an inner peripheral surface of the center hole of the inner shaft are brought into spline engagement with a spline formed on an outer peripheral surface of the smaller-diameter portion of the drive shaft, so that the inner shaft and the drive shaft are assembled together in the rotating direction. Also, an inner side surface of the inner ring is caused to abut against an outer side surface of the larger-diameter portion, and a nut is threaded onto a screw portion formed on an outer end portion of the smaller-diameter portion of the drive shaft, and by doing so, the drive shaft is pressed toward the outer side relative to the inner shaft, thereby connecting the inner shaft and the drive shaft together in the axial direction. By thus connecting the drive shaft to the inner shaft, etc., a suitable preload is applied to the inner ring of the rolling bearing, and also the drive shaft, the inner shaft and the inner ring can be rotated in unison.
There is known another such wheel rolling bearing apparatus in which the above method of fixing the inner ring relative to the inner shaft is changed, and an inner ring is fixed to an inner shaft by caulking an inner-side shaft end of the inner shaft (This caulking will hereinafter be referred to as “shaft end caulking”). In this wheel rolling bearing apparatus, a suitable preload is applied to the inner ring of the rolling bearing by the shaft end caulking. Then, an inner side surface of the caulking portion is caused to abut against an outer side surface of a larger-diameter portion of a drive shaft, and the drive shaft and the inner shaft are connected together by the pressing effected by a nut so that the drive shaft, the inner shaft and the inner ring can be rotated in unison.
Incidentally, in a situation in which a vehicle is quickly started or is quickly turned, a large torsional torque often acts on the drive shaft of the above wheel rolling bearing apparatus. When a large torsional torque is applied to the drive shaft, a very small relative motion tends to develop in a circumferential direction at the abutting surfaces because of the difference in rigidity between the drive shaft and the inner shaft, etc., at a region between the spline and the abutting surfaces. At this time, a stick-slip phenomenon occurs in which the relative motion of the abutting surfaces will not occur until the torsional torque occurring at the abutting surfaces exceeds a frictional resistance due to a contact pressure produced by the pressing effected by the nut, and when the torsional torque exceeds the frictional resistance, the relative motion suddenly occurs, thereby releasing the energy. When such a stick-slip phenomenon occurs, an abnormal sound is generated by the suddenly-occurring relative motion of the abutting surfaces.
Therefore, there has been proposed a wheel rolling bearing apparatus as described in Japanese Utility Model Registration No. 2,532,672, in which a low frictional member is interposed between abutting surfaces. In this wheel rolling bearing apparatus, the low frictional member enables the abutting surfaces to easily slip so as to suppress the occurrence of a stick-slip phenomenon, thereby suppressing the generation of an abnormal sound.
However, the wheel rolling bearing apparatus as disclosed in Japanese Utility Model Registration No. 2,532,672 is so constructed that the abutting surfaces can easily slip, and therefore there are fears that the rigidity in a connected condition of a drive shaft and an inner shaft, etc., may be lowered and that a fatigue strength of the drive shaft, etc., may be lowered. And besides, when wear of the drive shaft, etc., due to slip of the abutting surfaces progresses, there are fears that the strength of connection between the drive shaft and the inner shaft, etc., and a preload applied to the inner ring may be lowered.
Furthermore, with respect to an abnormal sound due to the above stick-sip phenomenon, the higher the contact pressure produced by the pressing by the nut becomes, the larger the energy released by the relative motion becomes, so that the sound pressure level increases. Therefore, in the case where attention is directed only to the coefficient of friction without considering the magnitude of the contact pressure as described in Japanese Utility Model Registration No. 2,532,672, the sound pressure level of the abnormal sound can not always be effectively reduced.